In the background art, there has been known an electrophotographic image forming apparatus including: a light source, which emits a light beam; a polygon mirror, which deflects the light beam; a motor, which rotates the polygon mirror; and a photosensitive member in which an image is formed by the light beam scanning the photosensitive member through the polygon mirror. In addition, a technique of controlling a motor, which rotates a polygon mirror is disclosed in the background art. The background art discloses an image forming apparatus that switches rotation control of a motor between control using a position signal (FG signal) of a rotor and control using a light receiving signal (BD signal) output from a sensor, which receives a light beam.